1. Field of the Invention
The present invention relates to density measurement of magnetic substances, in particular to a method and apparatus for determining concentration of a trace of contaminant magnetic substances suspended or contained in a non-magnetic substance by a superconducting quantum interference device (SQUID).
2. Description of the Related Art
FIG. 1 is a flow diagram illustrating a process for producing ceramic articles in which material powders purchased on market are subjected to a variety of severe examinations to check their qualities. In practice, the material powders are dispersed in water or alcohol to prepare a slurry in the step A. In the step B, the slurry is stirred, for example by a propeller type stirrer to prepare finer powders. Water or alcohol is evaporated in the step C by so-called spray-dry technique to obtain a conditioned powder which can be used in subsequent steps including molding or shaping (D), preliminary sintering (E) and final sintering (F) to obtain a desired final product of ceramics.
Material ceramic powders contains magnetic substances such as iron and nickel as impurities. Magnetic substance such as iron is often introduced in their manufacturing process for example in a form of solid solution in the material powders. In fact, powder of silicon nitride available on market contain iron as an impurity. This fact is described in a purity analysis table attached to commercially available powders.
The content of magnetic substance such as iron in material powders which exist microscopically in grain boundary or in grain itself is very low of ppm order but must be known or evaluated precisely so as to control the quality of ceramic products. In practice, it is necessary to know precise content of magnetic substances such as iron contained in material powders at as an earlier stage as possible, preferably before the dispersion stage B in the production process by measuring the density of magnetic fluid at high precision for example.
In a process for treating metal powders such as aluminum powder, iron is an undesirous contaminant impurity. Therefore, in this case also, it is requested to determine the density of magnetic fluid at high precision to know the precise content of magnetic substance such as iron contained in material powders.
Heretofore, the density (content) of magnetic elements such as iron contained in non-magnetic substances or material powders as contaminant impurities has been determined mainly by chemical analysis in which objective magnetic substances are extracted firstly and are determined by Inductively Coupled Plasma-atomic Emission Spectroscopy or by chemical analysis. In any ways, objective substance is dissolved firstly in a solution to determine the density of magnetic substances.
This conventional destructive analysis system, however, requires sophisticated procedure including sampling work of the objective substance followed by pre-treatment for analysis, so that it takes at least several hours to analysis and analysis can not be effected continuously.
Non-destructive analysis system is already known to determine physical value correlated to magnetic material which is contained at relatively high concentration or proportion in a liquid substance. Japanese patent publication No. JP-A-6-204,068 discloses an apparatus for determining easily and rapidly the water content in a magnetic material in a slurry form of oxide powder on line.
JP-A-7-5,137 discloses an apparatus for trapping iron particles magnetically in a circulating fluid and JP-A-7-260,912 discloses a technique to determine the density of saturated magnetic flux by means of a permanent magnet and two sample holders. In these patents, physical values correlated to relatively high concentration or proportion of magnetic component such as iron particles or iron powder are determined.
In these known techniques in which magnetic properties which magnetic material or iron powder possess are utilized, relatively strong magnetic force of magnetic material or iron powder itself is detected by a coil or by means of hall effect device in a form of inductance change. Sensitivity of these detectors, however, can not be increased to higher levels because too sensitive pulse-like response from individual magnetic particles disturb measurement of objective substance, so that these techniques can not be used to determine the concentration of a trace of contaminant magnetic substances dispersed uniformly in a non-magnetic substance.
In fact, chemical analysis has been only means that can be used to determine the precise concentration of a trace of magnetic substance contained in a non-magnetic substance.
An object of the present invention is to provide non-destructive method and apparatus for determining the density of a trace of magnetic substance such as iron contained in a non-magnetic substance by using a superconducting quantum interference device (SQUID) as a magnetic sensor at high precision and in non-contacted mode.
Another object of the present invention is to provide method and apparatus for measuring continuously the density of magnetic substance, which can be used in a variety of production processes as a monitor.
Still another object of the present invention is to provide a measuring method and apparatus of the density of magnetic substance, which can be operated in line without delay.